The present disclosure relates to gas turbine engines, and more particularly to circumferential grooves under a layer of abradable material to retain compressor stability performance associated with tight clearances late into the engine overhaul cycle.
In a gas turbine engine, air is compressed in various fan and compressor stages by rotor blades which cooperate with stator vanes. Fan air provides primary bypass propulsion thrust while compressor air is mixed with fuel and ignited for generation of hot combustion gases from which energy is extracted by turbine stages which power the compressor section and fan section.
Compressor blade tip clearances are a significant component of desirable performance as defined by fuel efficiency, and compressor stability as defined by stall margin. During certain transient conditions of the engine, differential expansion or contraction, or other radial movement between the engine casing and the blades may cause intermittent blade tip rubbing against the engine casing. Blade tip rubbing generates abrasion and friction heat that may subject the blade tips and engine casing to locally high stress. Blade tip rubbing may be reduced or eliminated by an increase of the nominal blade tip clearance, but this may result in a corresponding decrease in desirable performance and compressor stability. Maintenance of desirable performance and compressor stability is thus a tradeoff between blade tip clearance and the potential for blade tip rubbing.
One system that facilitates efficient engine operation is a rub strip. Rub strips include abradable coatings within the engine case. The abradable coating is at least partially eroded during engine break-in to provide efficient performance and compressor stability throughout a majority of the engine overhaul cycle. The abradable coating within the rub strip is relatively soft enough to protect the blade tips during regular operation but generally too soft to survive over a prolonged time period or from an isolated unanticipated rub event. Erosion of the rub strip increase the blade tip clearances that adversely affect both performance and compressor stability over time.
Another system that facilitates engine operation is a plurality of circumferential grooves disposed in the inner surface of the engine casing. When the rotor blades operate efficiently, airflow is pumped from the lower-pressure region forward of the rotor blades to the higher pressure region behind the rotor blades. Stall may occur when air leaks from the aft higher-pressure region, over the tip, to the forward lower-pressure region. The circumferential grooves assures effective compressor stability over the engine overhaul life cycle at the tradeoff of relatively less desirable performance as defined by fuel efficiency.